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| 型號(hào)： | EVAL-AD7751EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Energy Metering IC With On-Chip Fault Detection |
| 中文描述： | 電能計(jì)量IC，帶有片上故障檢測(cè) |
| 文件頁(yè)數(shù)： | 15/16頁(yè) |
| 文件大小： | 249K |
| 代理商： | EVAL-AD7751EB |
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REV. A
AD7751
–15–
Table III.
Max Frequency
for DC Inputs (Hz)
Max Frequency
for AC Inputs (Hz)
S1
S0
0
0
1
1
0
1
0
1
0.68
1.36
2.72
5.44
0.34
0.68
1.36
2.72
Frequency Output CF
The pulse output CF (Calibration Frequency) is intended for
use during calibration. The output pulse rate on CF can be up
to 128 times the pulse rate on F1 and F2. The lower the F
1–4
frequency selected the higher the CF scaling. Table IV shows
how the two frequencies are related depending on the states of
the logic inputs S0, S1, and SCF. Because of its relatively high
pulse rate, the frequency at this logic output is proportional to
the instantaneous real power. As is the case with F1 and F2,
the frequency is derived from the output of the low-pass filter
after multiplication. However, because the output frequency is
high, this real-power information is accumulated over a much
shorter time. Hence less averaging is carried out in the digital-
to-frequency conversion. With much less averaging of the real-
power signal, the CF output is much more responsive to power
fluctuations (see Signal Processing Block in Figure 2).
Table IV.
F
1–4
(Hz)
CF Max for AC Signals
(Hz)
128
×
F1, F2 = 43.52
64
×
F1, F2 = 21.76
64
×
F1, F2 = 43.52
32
×
F1, F2 = 21.76
32
×
F1, F2 = 43.52
16
×
F1, F2 = 21.76
16
×
F1, F2 = 43.52
8
×
F1, F2 = 21.76
SCF
S1
S0
1
0
1
0
1
0
1
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
1.7
1.7
3.4
3.4
6.8
6.8
13.6
13.6
SELECTING A FREQUENCY FOR AN ENERGY METER
APPLICATION
As shown in Table II, the user can select one of four frequencies.
This frequency selection determines the maximum frequency on
F1 and F2. These outputs are intended to be used to drive the
energy register (electromechanical or other). Since only four
different output frequencies can be selected, the available fre-
quency selection has been optimized for a meter constant of
100 imp/kWhr with a maximum current of between 10 A and
120 A. Table V shows the output frequency for several maxi-
mum currents (I
MAX
) with a line voltage of 220 V. In all cases
the meter constant is 100 imp/kWhr.
Table V.
I
MAX
12.5 A
25 A
40 A
60 A
80 A
120 A
F1 and F2 (Hz)
0.076
0.153
0.244
0.367
0.489
0.733
where,
Freq
V
1
V
2
Gain
= Output frequency on F1 and F2
(
Hz
)
= Differential rms voltage signal on Channel 1 (volts)
= Differential rms voltage signal on Channel 2 (volts)
= 1, 2, 8, or 16, depending on the PGA gain selection
made using logic inputs G0 and G1
= The reference voltage (2.5 V
±
8%) (volts)
= One of four possible frequencies selected by using the
logic inputs S0 and S1 (see Table II)
V
REF
F
1–4
Table II.
S1
S0
F
1–4
(Hz)
XTAL/CLKIN
*
3.579 MHz/2
21
3.579 MHz/2
20
3.579 MHz/2
19
3.579 MHz/2
18
0
0
1
1
0
1
0
1
1.7
3.4
6.8
13.6
*
F
are a binary fraction of the master clock and will thus vary if the specified
CLKIN frequency is altered.
Example 1
If full-scale differential dc voltages of +660 mV and –660 mV are
applied to V1 and V2 respectively (660 mV is the maximum
differential voltage that can be connected to Channel 1 and
Channel 2), the expected output frequency is calculated as follows.
Gain
=
1, G0 = G1 = 0
F
1–4
=
1.7 Hz, S0 = S1 = 0
V1
=
+660 mV dc = 0.66 volts (rms of dc = dc)
V2
=
–660 mV dc = 0.66 volts (rms of dc = |dc|)
V
REF
=
2.5 V (nominal reference value).
NOTE: If the on-chip reference is used, actual output frequencies
may vary from device to device due to reference tolerance of
±
8%.
Freq
Hz
Hz
=
×
×
=
5 74
.
0 66
.
0 66 1 1 7
2 5
.
0 68
.
2
.
.
(8)
Example 2
In this example, if ac voltages of
±
660 mV peak are applied to
V1 and V2, the expected output frequency is calculated as follows.
Gain
= 1, G0
=
G1
=
0
F
1–4
= 1.7 Hz, S0
=
S1 = 0
V1
= rms of 660 mV peak ac = 0.66/
√
2
volts
V2
= rms of 660 mV peak ac = 0.66/
√
2
volts
V
REF
= 2.5 V (nominal reference value).
NOTE: If the on-chip reference is used, actual output frequen-
cies may vary from device to device due to reference tolerance
of
±
8%.
Freq
Hz
Hz
=
×
×
×
×
2 5
.
×
×
=
5 74
.
0 66
.
0 66
.
1
1 7
.
2
2
0 34
.
2
(9)
As can be seen from these two example calculations, the maxi-
mum output frequency for ac inputs is always half of that for dc
input signals. Table III shows a complete listing of all maximum
output frequencies.
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